Air and other gaseous feed streams to industrial processes often contain trace level contaminants that can be detrimental to the end use of the feed gas or its separated components. In addition, processes using these purified gas streams are often sensitive to even low levels of these contaminants such that the purified product stream must be of high purity or ultra-high purity (UHP). The low concentration and chemical nature of contaminants in the feed gas, coupled with the need for high purity, often require the use of highly selective adsorbent or catalyst materials for contaminant removal. While some materials can be effective, the cost associated with these highly selective materials is often quite high (e.g., >$10/lb to exceeding $100/lb). In many instances only a thin layer (e.g., several inches) of such highly selective adsorbents or catalysts is required.
Distributing highly selective and expensive materials in commercial scale vessels and maintaining a uniform layer depth of only a few inches over the entire flow area of the vessel is difficult. Variations in layer depth result in premature breakthrough of the contaminant where the layer thickness is too thin. Achieving a high or ultra-high purity product stream under such a condition may then be unreliable. When using low to moderate cost adsorbents or catalysts (≦$10/lb), one solution is to increase the depth of the active layer. Adding more of an expensive material, however, may be prohibitive to the competitiveness of the process.
U.S. Pat. No. 5,258,060 to Gaffney et al. describes a bulk separation process for separation of nitrogen from air to produce oxygen. The active phase of adsorbent, with a high heat of adsorption, is diluted with an inert material in the range of 5% to 80% to reduce temperature swings and increase the effective N2 working capacity.
A mixture of weak and strong adsorbents in two different types of PSA processes is disclosed in U.S. Pat. No. 6,027,548 to Ackley et al. In the bulk separation of air to produce O2, both adverse thermal swing and thermal gradients are reduced by mixing adsorbents of high and low selectivity toward N2.
U.S. Pat. No. 4,499,208 to Fuderer relates to activated carbon doped with inert dense alumina and a reduced thermal swing when adsorbing CO2 at high pressure from a feed stream containing H2, CO2, CO and CH4.
Mixtures of fine and course particles have been applied to reduce interparticle void space, increase adsorbent density and increase gas storage capacity. Kaplan et al. (European Application No. 0 325 392) provides an example of this methodology applied in PSA systems employing carbon molecular sieve (CMS) adsorbents for kinetic separation of air to produce N2. U.S. Pat. No. 4,762,537 to Fleming et al. relates to a composite adsorbent produced by agglomerating a mixture of 50-95 wt % alumina and 5-50 wt % type Y zeolite formulated for removal of HCl present at 100 ppm or less from gas mixtures.
Heinze et al. (U.S. Pat. No. 3,773,690) discloses a binderless composite adsorbent comprising a mixture of type X and type A zeolites and the method of producing same.
A mixture of adsorbent and catalyst particles is contemplated in processes combining reaction and separation in a pressure swing reactor (PSR) (Alpay, et al. “Combined Reaction and Separation in Pressure Swing Processes,” Chem. Eng. Sci. 49, 5845-5864, 1994).
Prior art techniques have been primarily aimed at bulk separation or purification of contaminants of high concentration (>1000 ppm) where the use of mixtures of adsorbents has been motivated by reducing the adverse effects of thermal swing and/or thermal gradients. Adsorption of high concentrations from a gas stream as in the prior art typically results in the formation of an equilibrium zone and a mass transfer zone (MTZ), as is well known by those of ordinary experience in the art. The saturated equilibrium zone represents a much higher capacity of contaminant than can be achieved in the MTZ. Effective processes of this type strive to achieve an overall bed or layer thickness that is several times the size of the MTZ so as to maximize the productivity of the adsorbent (Wankat, P. C., Large-Scale Adsorption and Chromatography, Vol. 1, pgs 50-60, 1986). Prior art strategies to achieve such productivity enhancement include decreasing the MTZ by using smaller adsorbent particles or increasing the overall bed length. Little attention has been given in the prior art to trace contaminant removal using thin layers containing mixtures of adsorbent or catalyst.
It would thus be desirable to provide the ability to use highly selective expensive materials in relatively thin layers for the purification of feed gas streams having low levels of contaminants to produce high and ultra-high purity (UHP) product gases.